Adenosine is known to affect adenylate cyclase activity in various parts of the body. Since adenosine receptors can be characterized as A.sub.1 receptors (which inhibit adenylate cyclase) and A.sub.2 receptors (which increase adenylate cyclase activity), the need arose for a screening technique which could measure affinities of various drugs for these receptors.
Since selectivity for A.sub.2 adenosine receptors is characteristic of certain neuroloptic drugs, the isolation of A.sub.2 receptors is an important step in the screening of drugs, particularly neuroleptic and antipsychotic drugs. Tritiated 5'-N-ethyl carboxamide adenosine, [.sup.3 H]NECA, is a labelled ligand which is known to stimulate adenylate cyclase and, as such, has a high affinity for adenosine receptors. However [.sup.3 H]NECA will interact with both A.sub.1 and A.sub.2 receptors. Accordingly, it became desirable to find a method and reagent via the use of which A.sub.2 receptors could be assayed without significant interference from the presence of A.sub.1 receptors. Researcher Siu-Mei Helena Yeung and Richard D. Green have investigated the binding of N.sup.6 -cyclohexyl adenosine (CHA) in rat brain striatum containing labelled 5'-N-ethyl carboxamide adenosine. Their work was published in Naunyn-Schmiedeberg's Arch. Pharmacol., 325:218-225 (1984).